Java Enterprise Best Practices

Use Parameter Entities

The natural extension of using entity references in an XML document is using parameter entities in a DTD. A parameter entity looks very much like an entity reference:

<!ENTITY % common.attributes
    'id        ID    #IMPLIED
     account   CDATA #REQUIRED'
>

Here, a more strict textual replacement occurs. In this case, the common.attributes definition can be used to specify two attributes that most elements in a constraint set should have. You can then define those elements in the DTD, as shown in Example 5-2.

Example 5-2. Sample DTD with parameter entities

<!ELEMENT purchaseOrder (item+, manufacturer, purchaser, purchaseInfo)>
<!ATTLIST purchaseOrder %common.attributes;>
   
<!ELEMENT item (price, quantity)>
<!ATTLIST item %common.attributes;>
   
<!ELEMENT manufacturer (#PCDATA)>
<!ATTLIST manufacturer %common.attributes;>
   
<!ELEMENT purchaser (#PCDATA)>
<!ATTLIST purchaser %common.attributes;>
   
<!ELEMENT purchaseInfo (creditCard | check | cash)>

In Example 5-2, each element uses the common.attributes parameter entity, which will be converted into the string in the example (including the id and account attributes). This is done for each attribute list. And, like entity references, changing the value of the parameter entity changes the definitions for all elements. Again, this technique can be used to clean up the organization of your DTDs.